A lubricating oil used for automatic transmissions or continuously variable transmissions or internal combustion engines has been required to be improved in various durabilities such as thermal oxidation stability, anti-wear properties, and anti-fatigue properties and low temperature viscosity characteristics such as low temperature viscosity reduction and low temperature fluidity improvement. In order to improve these properties, a lubricating oil has been used, which comprises a base oil blended with various additives such as anti-oxidants, detergent dispersants, anti-wear agents, friction modifiers, seal swelling agents, viscosity index improvers, anti-foaming agents, and dyes.
Recent transmissions and engines have been demanded to be light and small and increased in power output, and in particular transmissions have been sought to be improved in power transmission capability in connection with the increased power output of the engines with which the transmissions are used in combination. Therefore, the lubricating oil used for such transmissions and engines have been required to have properties to prevent wear or fatigue on the surfaces of the bearings and gears while maintaining a higher level of lubricating performance. Continuously variable transmissions are also increased in torque transmitted between the metal pulleys and metal belt due to the increased power output of the engines. Therefore, the lubricating oil used for such transmissions have been required to have properties to prevent wear or fatigue on the metal surfaces. Further, automatic transmissions and continuously variable transmissions are supposed to be used in a cold region of −10° C. or lower and are thus required to be further improved in low temperature performances so as to enhance the low temperature startability and improve the fuel efficiency at low temperatures. Generally, the low temperature viscosity characteristics of a lubricating oil can be improved by reducing the viscosity of the base oil or final product. However, it is known that a reduction in the base oil viscosity degrades the anti-wear properties and anti-fatigue properties. The development of a lubricating oil has been eagerly desired which has both low temperature viscosity characteristics and anti-wear properties or anti-fatigue properties.
It has been known that an attempt to improve both fatigue life and low temperature characteristics were made using a base oil with a good low temperature performance or a base oil with a high viscosity in combination, or blending a phosphorus- or sulfur-based extreme pressure additive in a suitable amount (see, for example, Japanese Patent Laid-Open Publication Nos. 2004-262979, 11-286696, and 2003-514099).
However, the foregoing fails to attain all viscosity temperature characteristics and low temperature performance, and metal fatigue life sufficiently. Therefore, it has been demanded to develop a lubricating oil composition having all of these performance characteristics but having no problem in other performances.